In recent years, various transmission channel allocation methods are proposed in order to effectively utilize frequencies in the abruptly developing mobile communication system such as a portable telephone set, and some of these methods are put into practice.
FIGS. 12(a) to 12(c) illustrate arrangements of channels in various types of communication systems, i.e., a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system and a spatial division multiple access (SDMA) system.
The SDMA system is also referred to as a PDMA (path division multiple access) system.
The FDMA, TDMA and SDMA systems are now briefly described with reference to FIGS. 12(a) to 12(c). FIG. 12(a) illustrates the FDMA system, in which analog signals of users 1 to 4 are frequency-divided with radio waves of different frequencies f1 to f4 and transmitted so that the signals of the users 1 to 4 are separated by a frequency filter.
In the TDMA system shown in FIG. 12(b), digitized signals of respective users are time-divided with radio waves of different frequencies f1 to f4 every constant time (time slot) and transmitted so that the signals of the respective users are separated by a frequency filter and time synchronization between a base station and mobile terminal units of the respective users.
On the other hand, the SDMA system is recently proposed in order to improve frequency utilization efficiency of radio waves due to popularization of portable telephone sets. This SDMA system is employed for spatially dividing a time slot at the same frequency and transmitting data of a plurality of users, as shown in FIG. 12(c). In this SDMA system, signals of the respective users are separated through a frequency filter, time synchronization between a base station and mobile terminal units of the users and a mutual interference eliminator such as an adaptive array.
FIG. 13 is a schematic block diagram showing the structure of a transmission/receiving system 200 of a conventional SDMA base station.
In the structure shown in FIG. 13, four antennas #1 to #4 are provided for identifying users PS1 and PS2.
In a receiving operation, outputs of the antennas are supplied to an RF circuit 2101, amplified through a receiving amplifier, frequency-converted with a local oscillation signal, thereafter subjected to removal of unnecessary frequency signals through a filter and A/D converted in the RF circuit 2101, and supplied to a digital signal processor 2102 as digital signals.
The digital signal processor 2102 is provided with a channel allocation reference calculator 2103, a channel alloter 2104 and an adaptive array 2100. The channel allocation reference calculator 2103 previously calculates whether or not the adaptive array can separate signals received from the two users. In response to the result of this calculation, the channel alloter 2104 supplies channel allocation information including user information for selecting frequencies and times to the adaptive array 2100. The adaptive array 2100 weights the signals from the four antennas #1 to #4 in real time on the basis of the channel allocation information, thereby separating only a signal from a specific user.
[Structure of Adaptive Array Antenna]
FIG. 14 is a block diagram showing the structure of a transmission/receiving part 2100a corresponding to a single user in the adaptive array 2100. In the example shown in FIG. 14, n input ports 2020-1 to 2020-n are provided for extracting the signal of the desired user from input signals including a plurality of user signals.
Signals input in the input ports 2020-1 to 2020-n are supplied to a weight vector control part 2011 and multipliers 2012-1 to 2012-n through switching circuits 2010-1 to 2010-n. 
The weight vector control part 2011 calculates weight vectors w1i to wni with the input signals, a training signal, previously stored in a memory 2014, corresponding to the signal from the specific user and an output of an adder 2013. The subscript i indicates that the weight vector is employed for transmission/receiving to/from an i-th user.
The multipliers 2012-1 to 2012-n multiply the input signals received from the inputs ports 2020-1 to 2020-n by the weight vectors w1i to wni respectively, and supply the results to the adder 2013. The adder 2013 adds up the output signals from the multipliers 2012-1 to 2012-n and outputs the sum as a received signal SRX(t), which is also supplied to the weight vector control part 2011.
The transmission/receiving part 2100a further includes multipliers 2015-1 to 2015-n receiving an output signal STX(t) from an adaptive array radio base station, multiplying the same by the weight vectors w1i to wni supplied from the weight vector control part 2011 respectively and outputting the results. The outputs of the multipliers 2015-1 to 2015-n are supplied to the switching circuits 2010-1 to 2010-n respectively. In other words, the switching circuits 2010-1 to 2010-n supply the signals received from the input ports 2020-1 to 2020-n to a signal receiving part 1R when receiving the signals, and supply signals received from a signal transmission part 1T to the input/output ports 2020-1 to 2020-n when transmitting the signals.
[Operation Principle of Adaptive Array]
The operation principle of the transmission/receiving part 2100a shown in FIG. 14 is now briefly described.
In the following description, it is assumed that the number of antenna elements is four and the number PS of users simultaneously making communication is two, in order to simplify the illustration. In this case, signals supplied from the respective antennas to the receiving part 1R are expressed as follows:RX1(1)=h11Srx1(t)+h12Srx2(t)+n1(t)  (1)RX2(1)=h21Srx1(t)+h22Srx2(t)+n2(t)  (2)RX3(1)=h31Srx1(t)+h32Srx2(t)+n3(t)  (3)RX4(1)=h41Srx1(t)+h42Srx2(t)+n4(t)  (4)where RXj(1) represents the signal received in the j-th (j=1, 2, 3, 4) antenna, and Srxi(t) represents the signal transmitted from the i-th (i=1, 2) user.
Further, hji represents the complex coefficient of the signal transmitted from the i-th user and received in the j-th antenna, and nj(t) represents noise included in the j-th received signal.
The above equations (1) to (4) are expressed in a vector form as follows:X(t)=H1Srx1(t)+H2Srx2(t)+n1(t)  (5)X(t)=[RX1(t), RX2(t) . . . , RXn(t)]T  (6)Hi=[H1i, h2i, . . . , hni]T, (i=1, 2)  (7)N(t)==[n1(t), n2(t), . . . , nn(t)]T  (8)
In the equations (6) to (8), [ . . . ]T represents inversion of [ . . . ].
X(t) represents an input signal vector, Hi represents the coefficient vector of the signal received from the i-th user, and N(t) represents a noise vector respectively.
As shown in FIG. 14, the adaptive array outputs the signal synthesized by multiplying the signals received in the respective antennas by the weight coefficients w1i to wni as the received signal SRX(t). The number n of the antennas is four.
In order to extract a signal Srx1(t) transmitted from the first user, for example, under the aforementioned preparation, the adaptive array operates as follows:
An output signal y1(t) from the adaptive array 2100 can be expressed as follows by multiplying the input signal vector X(t) by a weight vector W1:y1(t)=X(t)W1T  (9)W1=[w11, w21, w31, w41]T  (10)
In other words, the weight vector W1 has a weight coefficient wji (j=1, 2, 3, 4) multiplied by the j-th input signal RX1(t) as an element.
The input signal vector X(t) expressed in the equation (5) is substituted in the input signal vector y1(t) expressed in the equation (9) as follows:y1(t)=H1W1TSrx1(t)+H2W1TSrx2(t)+N(t)W1T  (11)
When the adaptive array 2100 ideally operates, the weight vector control part 2011 sequentially controls the weight vector W1 by a known method to satisfy the following simultaneous equations:H1W1T=1  (12)H2W1T=0  (13)
When the weight vector W1 is completely controlled to satisfy the equations (12) and (13), the output signal y1(t) from the adaptive array 2100 is finally expressed as follows:y1(t)=Srx1(t)+N1(t)  (14)N1(t)=n1(t)w11+n2(t)w21+n3(t)w31+n4(t)w41  (15)
In other words, it follows that the signal Srx1(t) transmitted from the first user in the two users is obtained in the output signal yt(t).
Referring to FIG. 14, the input signal STX(t) for the adaptive array 2100 is supplied to the transmission part 1T in the adaptive array 2100, to be supplied to the first inputs of the multipliers 2015-1, 2015-2, 2015-3, . . . , 2015-n. The weight vectors w1i, w2i, w3i, . . . , wni calculated by the weight vector control part 2011 on the basis of the received signals as described above are copied and applied to the second inputs of these multipliers respectively.
The input signals weighted by these multipliers are sent and transmitted to the corresponding antennas #1, #2, #3, . . . , #n through the corresponding switches 2010-1, 2010-2, 2010-3, . . . , 2010-n.
The users PS1 and PS2 are identified in the following manner. A radio signal from a portable telephone set is transmitted in a frame structure. The radio signal from the portable telephone set is roughly formed by a preamble consisting of a signal series known to the radio base station and data (voice etc.) consisting of a signal series unknown to the radio base station.
The signal series of the preamble includes a signal string of information for identifying whether or not this user is a desired user for making communication with the radio base station. The weight vector control part 2011 of the adaptive array radio base station 1 compares the training signal corresponding to the user A fetched from the memory 2014 with the received signal series and performs weight vector control (decision of a weight coefficient) to extract a signal seeming to include the signal series corresponding to the user PS1.
In general, QPSK modulation or the like which is a modulation system based on PSK modulation is employed as a modulation system applied to transmission/receiving in a portable telephone or the like.
In the PSK modulation, synchronous detection performing detection by integrating a signal synchronous with a carrier on a received signal is generally carried out.
In the synchronous detection, a local oscillator generates a complex conjugate carrier synchronized with the center frequency of a modulated wave. When the synchronous detection is performed, however, frequency errors referred to as frequency offsets are generally present in oscillators on the transmission end and the receiving end. When the received signal is expressed on an IQ plane, the position of the received signal point is rotated on the receiver side due to the errors. Therefore, it is difficult to perform the synchronous detection unless compensating for the frequency offsets.
Such frequency offsets are generated not only by the precision of a local oscillation frequency in the aforementioned transmission/receiving period but also by a set error, temperature fluctuation, time change and the like, and the receiving characteristic is abruptly deteriorated due to a carrier frequency component remaining in the signal input in the receiver.
A technique of providing the so-called “automatic frequency control function (AFC)” in the communication system is known as a method of suppressing such carrier frequency offsets. In such a generally performed automatic frequency control function, however, there is the possibility that a sufficient operation cannot be expected under mobile communication having a transmission condition such as wide band modulation, high-speed phasing, burst signal transmission, multi-path delay distortion, common-frequency interference or the like.